A solid electrolytic capacitor is a capacitor in which an anode body comprising metal having a valve action or the like is subjected to a chemical formation to form an oxide in a surface layer of the anode body and the surface layer is utilized as a dielectric layer. Tantalum or aluminum is often used in an anode body of a commercially available solid electrolytic capacitor. An aluminum electrolytic capacitor is suited for smoothing a power supply circuit, a time constant circuit and the like since a large capacity is obtained. A tantalum electrolytic capacitor is a miniature electrolytic capacitor and is excellent in characteristics as compared with the aluminum electrolytic capacitor. In particular, the tantalum electrolytic capacitor is often used in an analog circuit. It is also used in a digital circuit for the purpose of removing a spike-shaped current.
Niobium metal is known as metal having physical or chemical properties which are similar to those of tantalum metal. Since reserves of niobium are larger than those of tantalum, stabilization of supply and cost reduction of an electrolytic capacitor can be expected of niobium. There is a possibility that the capacity of the niobium electrolytic capacitor becomes larger than that of the tantalum electrolytic capacitor. Therefore, it is expected that the tantalum electrolytic capacitor will be replaced by the niobium electrolytic capacitor in the future.
However, when electrolytic formation of the anode body comprising niobium was carried out under chemical formation conditions similar to those in case of aluminum or tantalum, satisfactory characteristics could not be obtained. In other words, a niobium oxide film obtained by chemically forming an anode body made of niobium was unstable as compared with a tantalum oxide film. In particular, the niobium oxide is two times the tantalum oxide in a formed film thickness per a chemical formation voltage, and also the niobium oxide is two times the tantalum oxide in strain generated with the growth of the film. Therefore, the withstand voltage on the basis of the film thickness of the niobium oxide film was half that of the tantalum oxide film. In niobium oxide, a non-stoichiometric lower oxide, which does not exist in tantalum oxide, exists. It is considered that this non-stoichiometric lower oxide promotes diffusion of oxygen in a dielectric layer and imparts semiconductive properties to the dielectric layer, and thus causing an increase in the leakage current.
The niobium oxide film has such unstable characteristics. However, there is a possibility that the niobium electrolytic capacitor exhibits performances which are better than those of the tantalum electrolytic capacitor, and thus much more studies have been made.
For example, PLT 1 describes a method for producing an electrolytic capacitor, which comprises electrolytically forming a sintered body made of niobium at a temperature of about 40° C. or lower using, as a chemical forming solution, an aqueous solution containing at least one acid or a salt thereof selected from phosphoric acid, nitric acid, sulfuric acid, adipic acid, boric acid and salts thereof, as a solute.
PLT 2 describes a method for producing an electrolytic capacitor, which comprises electrolytically forming an anode body comprising niobium at a low temperature of about 15° C. using a chemical forming solution acidified by adding an acidifying agent composed of adipic acid to an aqueous solution containing a salt of boric acid or a salt of adipic acid.
PLT 3 describes a method for anodizing niobium for an electrolytic capacitor, which comprises electrolytically chemical forming pellets of metallic niobium in an aqueous phosphoric acid solution adjusted to pH 6 to 11.5, taking out the pellets from the chemical forming solution, immersing the pellets in a mixed solution of phosphoric acid and nitric acid for several minutes to about 10 minutes, subjecting to an annealing treatment comprising heating at a temperature of 250° C. to 800° C. for several minutes to about 10 minutes and slow cooling, and finally subjecting to restorative chemical formation in aqueous phosphoric acid solution adjusted to pH 0 to 5.9.
PLT 4 describes a method for producing a solid electrolytic capacitor, which comprises electrolytically chemical forming an anode body, which is obtained by sintering a valve action metal powder, in an aqueous phosphoric acid solution, and then electrolytically chemical forming it in an aqueous nitric acid solution at a voltage of 60 V or less. However, PLT 4 only discloses, as Examples, that the leakage current could be suppressed when a tantalum sintered body is subjected to a chemical formation.
PLT 5 describes a method for a chemical formation of a valve action metallic material, which comprises steps of chemically forming in an electrolytic solution containing adipate, chemically forming in an electrolytic solution containing at least one selected from the group consisting of oxalic acid, nitric acid, sulfuric acid, adipic acid, phosphoric acid, silicic acid and a salt thereof, heat-treating at a temperature of 250° C. to 400° C., and chemically forming in an electrolytic solution containing adipate in this order. However, PLT 5 only discloses, as Examples, that the leakage current could be suppressed when an aluminum foil is subjected to a chemical formation.